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Spotlight on Optics

August 2018

Spotlight Summary by James R. Taylor

The superior beam quality, high repetition rate, and high efficiency of fiber lasers, together with their competitive average power and short pulse capability, make them more attractive than their solid state free-space counterparts for numerous applications. However, the initiation of nonlinear effects at relatively modest peak powers severely limits their widespread deployment and development. The application of chirped pulse amplification technology to fiber-based schemes has allowed extracted pulse energies to approach the millijoule level, yet this is still significantly below the capability of the joule level pulses obtained from solid state systems. Divided pulse amplification schemes have been proposed and demonstrated where a pulse is split into numerous time-delayed replicas, amplified, and then coherently recombined using optical delay lines; however, scaling this to combine hundreds of pulses to obtain joules-scale pulses is made difficult by the complexity of the required optical delay line system.

In this report, the authors from Lawrence Berkeley National Laboratory and the University of Michigan describe a phase-stabilized coherent pulse stacking scheme that has the potential to combine in excess of one hundred pulses in a relatively simple cascaded delay line configuration. The basic delay line building block employs counter-propagating beams in a ring path with the optimized reflectivity of the input beam splitter depending on the number of cascaded sections and, hence, pulse energy enhancement. By cascading a sequence of interferometers of increasing loop lengths, the enhancement factor can be increased: for example, in a cascade of two cavities at the fundamental time period plus two at five times the fundamental, 25 pulses can be overlapped. The phase stability of the cavities strongly affects the efficiency of the recombination and stacking performance, and cavity lengths have to be controlled to nanometer accuracy to achieve about 1% stability. The authors employed a technique which they call modulated impulse response phase detection, whereby the cavity round trip phase was measured and a fast feedback control loop added to lock the cavity phase.

In the experimental implementation, the 10-ps pulses from a mode-locked Nd:YAG laser were stacked in the so-called 2+2 arrangement described above (two delay cavities at the fundamental, plus two at five times the fundamental time period). Long-term stability was demonstrated with the output stable to 1.5% for over 30 hours and a pulse energy enhancement of 18.4 recorded, comparing favorably to the theoretically predicted value of 21.5. The authors are confident that optimization of the delay cavities will lead to greater stability and enhancement, with the technique potentially leading to significantly enhanced pulse energies.